The device disclosed in this document relates to direct-current microgrids and, more particularly, to a direct-current power server for a direct-current microgrid.
A power distribution system in a commercial building is responsible for safely distributing electrical power to loads throughout the building. A typical power distribution system consists of components such as metering devices, protective devices (over-current, over-voltage, over-power, etc.), switching devices, transformers, controllers, and conductors. Within a typical commercial building, alternating-current (AC) power is distributed to loads through switchboards or panelboards. A panelboard is an enclosure for overcurrent protection devices for the busses and branch circuits that distribute power to building loads or their associated circuits, as defined in the National Electric Code. Power to lighting in a commercial building is commonly distributed through dedicated panelboards and power flow to the lighting is controlled by overcurrent protection devices on the corresponding branch circuits. Lighting panelboards are often wall mounted and their physical size and capacity ratings are standardized within the industry.
As renewable energy technology advances, many commercial building owners or lessees are considering deployment of renewable energy assets. However, large-scale deployment of distributed renewable energy will be achieved only when the renewable energy assets provide attractive returns to their owners, while also allowing utilities and grid operators to safely and reliably mitigate the impacts of intermittency on the power distribution infrastructure. Current systems for retrofitting a commercial building's power distribution system to utilize renewably energy assets have several disadvantages.
FIG. 1 shows a typical system 100 for integrating an on-site photo-voltaic (PV) system into a building's power distribution system. A PV array 104 is provided on-site to generate power for the building. The PV array 104 may, for example, be situated on the roof of the building. The PV array 104 generates a varying amount of direct-current (DC) power, depending on the time of day and weather. The PV array 104 is connected to an inverter 108, which converts the DC power to AC power. The inverter 108 is connected to the AC electrical grid 112 via the building's switchboard or panelboard. The building's power distribution system draws AC power directly from electrical grid 112. In order for the building's power distribution system to provide power to DC loads 116, the AC power drawn from the electrical grid 112 must be converted to DC power. Rectifiers 120 must be connected between the DC loads 116 and the building's power distribution system in order to provide DC power to the DC loads 116.
The system 100 has inefficiencies that cause the system 100 to be less cost-effective and, therefore, less attractive to building owners. The inverter 108 typically has conversion losses of around 3%-8%. These conversion losses can be even higher, depending on the design of the inverter 108 and on operating conditions, such as the weather. The inverter 108 also has increased losses when operating a partial capacity, such as at times of day during which less sunlight shines on the PV array 104. Additionally, as DC loads 116 become more prevalent in commercial buildings, conversion losses associated with the rectifiers 120 become increasingly relevant. Common DC loads in commercial buildings include solid state LED lighting, fluorescent lighting, IT equipment, electric vehicle chargers, DC motors, and motors with variable frequency drives (VFDs). Typical rectification losses for these loads range between 4%-25%. The conversions from DC to AC and from AC back to DC for powering DC loads results in a considerable reduction in the total amount of PV energy that is actually utilized by the building. In addition, the reliability of the system is reduced through the extra electronics required for DC to AC and AC to DC energy conversions.
The system 100 also cannot provide any power from the PV array 104 to the building during a power outage on the electrical grid 112. Since the PV array 104 is not directly connected to the building's power distribution system, electrical grid 112 is effectively in the path between the PV array 104 and the building loads. To enable delivery of on-site renewable energy to critical loads during grid outages, stationary energy storage devices having sophisticated grid-forming inverters and transfer switches must be provided in AC-based system 100 to operate the building's power distribution system and utilize the PV array 104 in an islanded mode.
Since the PV array 104 of the system 100 is tied directly to the electrical grid 112, the system 100 may be subject certain regulatory requirements and the building owner must obtain permission from the utility company that operates the electrical grid 112 before operating the system 100. Generally, the building owner must acquire an interconnection agreement with the utility company, which may subject the building owner to certain fees and other expenses.
What is needed is a power server that integrates renewable energy assets into a building's power distribution system, allowing the most efficient transfer of energy from sources to loads without direct connection to the electrical grid, such that the renewable energy assets can provide power to the building in isolation from the electrical grid.